000
FXAK68 PAFC 210142
AFDAFC
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 PM AKST Tue Feb 20 2018
.ANALYSIS AND UPPER LEVELS...
A look at the weather map shows a low pressure system moving near
the Bering Strait and into the Arctic Ocean. An associated
weather front extends southeastward from the low and has plowed
across virtually all of mainland Alaska in the last 24 hours.
While this storm has brought major impacts to the interior
portions of the state, our forecast area has had much lighter
impacts. In fact, as the front went through this morning, it
couldn`t even produce enough vertical mixing in the atmosphere to
churn up and get rid of the low clouds and fog stuck around upper
Cook Inlet and parts of the Bristol Bay zone and Upper Kuskokwim
Valley. With the leftovers of the front now moving away from us.
Low clouds and fog are likely to persist tonight as warm
advection overspreads the area once again from the southwest.
Over the Bering Sea and Aleutians, thick clouds and widespread
precip has already moved into the western 2/3 of the area as the
next low center is just about to cross over the western Aleutians
and into the Bering Sea. This storm will follow a similar track to
the last one and track northeastward toward the Bering Strait.
&&
.MODEL DISCUSSION...
The available models are in reasonably good agreement on the
overall path of the next storm through our AOR (the storm
currently moving into the western Aleutians). All solutions depict
a large and deep pocket of warm air moving into Southwest and then
Southcentral Alaska in the next 12-36 hours. As this warm sector
continues off to the east on Wednesday night/Thursday, a cold
front will surge eastward through Southwest and then Southcentral
Alaska. This cold front is the focus of most of our effort today,
and represents the forecast challenge of the day. The problem is
not with the surface features as much as the upper leveltrough/dynamics associated with the storm. As the supporting upper
trough passes through southern Alaska, there is disagreement on
the overall orientation of the feature. The NAM/GFS bring the
trough through with a neutral or even slightly positive tilt,
which would push the whole thing through fairly quickly and with
fairly minor amounts of precipitation in most areas. Meanwhile,
the Canadian and European models have tried to go neutral and then
even negatively tilted with the trough. This would result in much
heavier and more widespread precipitation across Southcentral on
Thursday morning than the NAM/GFS models. Forecast experience and
pattern recognition would support the NAM/GFS solutions, so that
is what the forecast is largely based on today, and some movement
toward this solution was noted in the 12z GEM/Canadian model run
this morning.
&&
.AVIATION...
PANC...IFRcig/vis will likely persist as warm air moving up Cook
Inlet tonight helps strengthen the temperature inversion.
Therefore the low clouds and/or fog will continue through the
Wednesday morning period with increasing mid level clouds
improving the situation by afternoon.
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Areas of low stratus and fog will linger across portions of
Southcentral and particularly the Cook Inlet region through
Wednesday morning as weak outflow winds quickly diminish and
upper level ridging is re-established over the mainland. This
will yield generally quiet conditions Wednesday into Wednesday
night before the next trough approaches from the west. This
feature will bring another round of winter precipitation to much
of the area early Thursday morning into Thursday afternoon,
although many of the details are still uncertain. A favorable
southwest flow pattern aloft yields the greatest forecast
confidence from Hatcher Pass into the northern Susitna Valley,
with a quick 6-12 inches possible in Hatcher Pass by Thursday
morning. Accumulations are expected to be more limited over the
Susitna Valley as the system looks to move through pretty quickly
given the progressive nature of the synoptic pattern. The
situation is a bit more muddled for Anchorage, as the presence of
southeasterly winds at the surface and temperatures hovering near
freezing will introduce precipitation type concerns. This lends
lower confidence to the forecast, with a rain/snow mix possibly
helping to limit accumulations to around one inch or less within
the Bowl. The trough will then swing east to Prince William Sound
and the Copper River Basin by Thursday afternoon, bringing snow to
these locations before departing to the east Thursday evening and
paving the way for another round of outflow winds to begin the mid
term part of the forecast.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Light southerly flow along with low stratus and fog will persist
over much of the southwestern Mainland this evening through
Wednesday morning. A weak upper level wave tonight will preface a
larger wave moving into the southwest Mainland Wednesday evening,
both of which will bring some precipitation and gusty onshore
winds to the area tonight through Thursday night. Precipitation
will likely start off as a mixture of snow and rain across the
southern mainland tonight, transitioning to mostly rain overnight
tonight into tomorrow morning. Snow will begin mixing back in with
rain through the remainder of the day Wednesday, and eventually
transition back to predominantly snow by Thursday morning as
colder air aloft begins to filter back into the area from the
west.
&&
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A stacked gale force low over the western Aleutians will slowly
track northward tonight. The associated front will continue to
track northeastward from the central Aleutians/Bering through the
eastern Aleutians/Bering tonight through Wednesday morning.
This system will not be nearly as strong as the previous, but it
will last longer since it is slower moving, so expect gale force
winds over parts of the Bering Sea through Wednesday before the
low weakens and moves toward the Southwest coast Wednesday night.
The next system enters the western waters Thursday morning,
bringing another round of precipitation and gusty winds to the
area.
&&
.LONG TERM FORECAST (Thursday evening through Tuesday)...
Expect the active pattern to continue with near to above normal
temperatures, periodic clouds/snow with dry and colder periods
between.
A short-lived dry outflow pattern is on tap for Thursday night as
the previous upper-level trough exits to the east. Dry, clear,
and cold conditions will prevail everywhere, as well as gusty gap
winds along the north gulf coast. These conditions will hold on
Friday, slowly weakening, as the next system pushes into the area
from the southwest overnight Friday into Saturday. Guidance is
struggling with how to track the triple point low as the front
moves into the Gulf, and hence how far north to track to frontal
boundary. The front should make it to the coast and the southern
Kenai Peninsula, bringing snow to these areas. A return to short-
lived outflow conditions comes late weekend when that system
exits.
Into the extended forecast, the upper-level southwesterly flow
remains. The progressive nature of the pattern will continue
through next week, with front and lows rolling into the region
every 1-2 days. At this point, the low amplitude pattern should
stop any big warm-up from happening, any warmer air associated
with the fronts should slide to the east with the triple-point
lows.
&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 173 175-179.
&&
$$
SYNOPSIS AND MODEL DISCUSSION...AMD
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...MTL